The best option would be to install from scratch, although losing all your files inside the computer during the process. Being an old computer, however, I assume this is not an issue.
Regarding issues with the ISO and the USB drive, two common issues are corrupted files and faulty hardware. To rule this out, check the integrity of the ISO after downloading it and try another USB drive.
If everything is correct thus far but you're still having issues, I would recommend downloading the ISO for Zorin OS 16 instead of the current release. It's still supported – until April, 2025 – and you can upgrade from there to Zorin OS 17 afterwards if you so wish to.
As a final measure if everything else fails, you can try to install Zorin OS over the network. You'll need another computer (this guide assumes it also runs Linux, but I'm sure a similar approach exists for Windows) and your laptop needs to have an ethernet port.